An Atmospheric General Circulation Model Assessment of Oceanic Impacts on Extreme Climatic Events over Japan in July 2018

Through a set of ensemble experiments with an atmospheric general circulation model (AGCM), potential influence of sea-surface temperature (SST) anomalies is assessed on large-scale atmospheric circulation anomalies that induced two extreme events observed over Japan in July 2018. One is a heavy rainfall event in early July mainly over western Japan, which was primarily caused by extreme moisture inflow associated with a cyclonic anomaly to the southwest of Japan and an anticyclonic anomaly to the east of Japan. An AGCM experiment with prescribed global SST anomalies cannot reproduce the anticyclonic anomaly, leading to the failure to simulate the enhancement of the moisture inflow and thereby precipitation over western Japan. The other extreme event is heat wave in mid- and late July almost over the entire Japan, which was caused by a strong anticyclonic anomaly around Japan. The AGCM experiment with global SST anomalies can well reproduce the warm anticyclonic anomalies. The additional experiments have confirmed that SST anomalies in both the tropics and midlatitude North Pacific have potential for forcing the leading mode of the atmospheric variability over the western North Pacific that brought the heat wave. Both the tropical and extratropical SST anomalies are also found to force a poleward shift of the subtropical jet axis over the western Pacific and anomalous tropospheric warming in the midlatitude Northern Hemisphere, both of which persisted in June and July.